The operating range of radio frequency power amplifiers is wider than that of small-signal amplifiers; hence, the power amplifiers are non-linear. For example, a base station in a cellular radio system simultaneously receives and amplifies signals of different frequencies from several terminals. In the prior art solutions, attempts have been made to correct signal distortion caused by power amplifier intermodulation by using feedforwarding or predistortion. In feedforward solutions, two control loops provided with a main amplifier for the actual signal and a distortion amplifier for signal distortion are typically applied. Distortion feedforwarding is here used for correcting the actual signal.
In the prior art solution employing predistortion, an estimate is made of the manner in which an amplifier will distort a signal. The signal to be amplified by means of the estimate is predistorted by a distortion transformation opposite to the amplifier distortion. The amplifier then, while amplifying the signal, simultaneously compensates for the predistortion, yielding an undisturbed, so-called linearized signal.
In the prior art solutions, predistortion is performed either analogically or digitally. It is difficult to detect changes in amplifier distortion in analogue predistortion; hence, digital predistortion is more advantageous. It is possible to correct distortion extremely efficiently by digital predistortion. A typical digital predistortion is performed by using lookup tables, which should, in addition, advantageously be updated in order to achieve adaptability since amplifier distortion is affected by temperature, the age of the amplifier and the changes of the signal fed to the amplifier, for example. U.S. Pat. No. 5,049,832, which is incorporated herein by reference, discloses one such solution. In the solution of the publication, predistortion information is stored in a memory in rectangular coordinate form, which aims to reduce the amount of information to be stored, and hence an attempt is made to quicken the adaptability of the solution in changing circumstances.
Typically, a problem of this kind of totally digital predistortion is slowness, however. The current digital signal processing technique enables the bandwidth to range from a few dozen to a maximum of a few hundred kilohertz when complex amplification predistortion is used. Bandwidth is even narrower when polar predistortion is used on account of numerous transformation computations. Furthermore, another problem of digital predistortion is the estimation of the delay caused by a power amplifier.